1. Field of the Invention
The present invention relates to a picture data reproducing apparatus and method, usable suitably for reproducing picture data coded according to the MPEG (Moving Picture Experts Group) standard by decoding the data.
This application claims the priority of the Japanese Patent Application No. 2002-206119 filed on Jul. 15, 2002, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Recently, various digital video coding techniques as typified by MPEG-2 (ISO/IEC 13818) have been proposed to compress a video by coding. The video compression according to the MPEG standard is such that a signal produced by effecting a hybrid transform which is a combination of inter-picture motion compensation and DCT (discrete cosine transform) undergoes quantization and variable-length coding.
According to the MPEG standard, picture planes (frames or fields) included in a video are compressed by coding them into any one of picture types including I, P and B pictures. The I picture is an intra-frame predictive coded picture or an intra coded picture. It is a picture predictively coded in one frame. The P picture is a frame forward-predictive coded picture or a predictive coded picture. It is a picture predicted by referring to a previous frame (I or P picture) which is already coded. The B picture is a bidirectionally predictive coded picture or a bidirectional coded picture. It is a picture predicted by referring to two frames, namely, a previous one and later one.
As above, according to the MPEG standard, video compression is done by inter-picture predictive coding, whereby it is made possible to efficiently compress a video and access the compressed video at random. Also, according to the MPEG standard, pictures of each of the above types are compressed into a data stream formed in units of a group of pictures (GOP) including an arbitrary number of pictures. The MPEG standard prescribes that at least one I picture should be included in a GOP to make it possible to make random access to a video compressed in units of GOP.
Note here that the MPEG-defined technique is not advantageous in that in case data is reproduced at a half, quarter, . . . , of a normal speed or in case a so-called variable-speed reproduction including frame advance and the like is shifted to the normal-speed reproduction, a skip is made over a picture being displayed due to the speed changing. Also, since picture cannot be displayed at the normal speed before reproduction of a recording medium is completely shifted to the one at the normal speed, shifting of the variable-speed reproduction to the desired normal-speed reproduction takes a long time before the user can actually view a video reproduced at the desired speed.